The invention relates to a method of coding video for a plasma display panel. More particularly, the invention relates to the coding of the grey levels of a type of panel with separate addressing and sustaining.
Plasma display panels, called hereafter PDPs, are flat-type display screens. There are two large families of PDPs, namely PDPs whose operation is of the DC type and those whose operation is of the AC type. In general, PDPs comprise two insulating tiles (or substrates), each carrying one or more arrays of electrodes and defining between them a space filled with gas. The tiles are joined together so as to define intersections between the electrodes of the said arrays. Each electrode intersection defines an elementary cell to which a gas space corresponds, which gas space is partially bounded by barriers and in which an electrical discharge occurs when the cell is activated. The electrical discharge causes an emission of UV rays in the elementary cell. Phosphors (red, green or blue) deposited on the walls of the cell convert the UV rays into visible light.
In the case of AC-type PDPs, there are two types of cell architecture, one called a matrix architecture and the other called a coplanar architecture. Although these structures are different, the operation of an elementary cell is substantially the same. Each cell may be in the ignited or xe2x80x9conxe2x80x9d state or in the extinguished or xe2x80x9coffxe2x80x9d state. A cell may be maintained in one of these states by sending a succession of pulses, called sustain pulses, throughout the duration over which it is desired to maintain this state. A cell is turned on, or addressed, by sending a larger pulse, usually called an address pulse. A cell is turned off, or erased, by nullifying the charges within the cell using a damped discharge. To obtain various grey levels, use is made of the eye""s integration phenomenon by modulating the durations of the on and off states using subscans, or subframes, over the duration of display of an image.
In order to be able to achieve temporal ignition modulation of each elementary cell, two so-called xe2x80x9caddressing modesxe2x80x9d are mainly used. A first addressing mode, called xe2x80x9caddressing while displayingxe2x80x9d, consists in addressing each row of cells while sustaining the other rows of cells, the addressing taking place row by row in a shifted manner. A second addressing mode, called xe2x80x9caddressing and display separationxe2x80x9d, consists in addressing, sustaining and erasing all of the cells of the panel during three separate periods. For more details concerning these two addressing modes, a person skilled in the art may, for example, refer to U.S. Pat. Nos. 5,420,602 and 5,446,344.
FIG. 1 shows the basic time division of the xe2x80x9caddressing and display separationxe2x80x9d mode for displaying an image. The total display time Ttot of the image is 16.6 or 20 ms, depending on the country. During the display time, eight subscans SC1 to SC8 are effected so as to allow 256 grey levels per cell, each subscan making it possible for an elementary cell to be xe2x80x9conxe2x80x9d or xe2x80x9coffxe2x80x9d for an illumination time Tec which is a multiple of a value To. Hereafter, reference will be made to an illumination weight p, where p corresponds to an integer such that Tec=p*To. The total duration of a subscan comprises an erasure time Tef, an address time Ta and the illumination time Tec specific to each subscan. The address time Ta can also be decomposed into n times an elementary time Tae, which corresponds to the addressing of one row. Since the sum of the illumination times Tec needed for a maximum grey level is equal to the maximum illumination time Tmax, we have the following equation: Ttot=m*(Tef+n*Tae)+Tmax, in which m represents the number of subscans. FIG. 1 corresponds to a binary decomposition of the illumination time. This binary representation has numerous drawbacks. A problem of false contours (or xe2x80x9ccontouringxe2x80x9d) has been identified for quite some time.
The problem of false contours stems from the proximity of two areas whose grey levels are very close but whose illumination instants are decorrelated. The worst case corresponds to a transition between the levels 127 and 128. This is because the grey level 127 corresponds to an illumination for the first seven subscans SC1 to SC7, while the level 128 corresponds to the illumination of the eighth subscan SC8. Two areas of the screen placed one beside the other, having the levels 127 and 128, are never illuminated at the same time. When the image is static and the observer""s eyes do not move over the screen, temporal integration takes place relatively well (if any flicker effect is ignored) and two areas with relatively close grey levels are seen. On the other hand, when the two areas move over the screen (and/or the observer""s eye moves), the integration time slot changes screen area and is shifted from one area to another for a certain number of cells. The shift in the eye""s integration time slot from an area of level 127 to an area of level 128 has the effect of integrating so that the cells are off over the period of one frame, which results in the appearance of a dark contour of the area. Conversely, shifting the eye""s integration time slot from an area of level 128 to an area of level 127 has the effect of integrating so that the cells are lit over the duration of one frame, which results in the appearance of a light contour of the area. This phenomenon is manifested, when working on pixels consisting of three elementary cells (red, green and blue), as false coloured contours.
The explained phenomenon occurs at all level transitions where the switched illumination weights are totally or almost totally different. Switchings of high weight are more annoying than switchings of low weight because of their magnitude. The resulting effect may be perceptible to a greater or lesser extent depending on the switched weights and on their positions. Thus, the contouring effect may also occur with levels that are quite far apart (for example 63-128), but it is less shocking for the eye as it then corresponds to a very visible level (or colour) transition.
To remedy this problem of contouring, several solutions have been implemented. One solution consists in xe2x80x9cbreaking upxe2x80x9d the high weights, this involving adding extra subscans. Only the total time of display of the image Ttot=m*(Tef+n*Tae)+Tmax remains fixed, thereby resulting in a drop in the time Tmax (since Tef and Tae are incompressible durations) and hence a drop in maximum brightness of the screen. It is possible to use up to 10 subscans while having correct brightness. With 10 subscans, the maximum illumination time Tmax is, currently, 30% of the total time whereas the erasure and address time is of the order of 70%. FIG. 2 represents an example of addressing using 10 subscans SC1 to SC10 in which the high weights are broken up into two.
In order to reduce the considerable transitions and to increase the number of subscans without reducing the brightness of the screen, one technique consists in simultaneously scanning two successive rows for certain illumination values. We then have the following equation Ttot=m1*(Tef+n*Tae)+m2*(Tef+n/2*Tae)+Tmax. The erasure time Tef being negligible relative to n*Tae, we have the equivalence Ttot≅(m1+m2/2)*(Tef+n*Tae)+Tmax. These simultaneous subscans halve the address time, and thus make it possible to add extra subscans without reducing Tmax. FIG. 3 represents an example of addressing with 11 subscans S1 to S11. Subscans S1 and S2 corresponding to the lowest illumination times are carried out on two rows at the same time so as to obtain an overall address time for these two subscans which is equal to the address time of a single subscan. If subscans common to two successive rows are performed for the illumination weights 1, 2, 4 and 8, it is possible to obtain 12 subscans so as to eliminate the transitions of weight 64. The problem with this solution is however the loss of resolution due to the simultaneous scanning of two rows.
With regard to the principle of subscans scanning two rows at the same time, one solution consists in using a rotating-code coding, or multiple-representation coding. FIG. 4 illustrates a rotating-code coding using twelve subscans S1 to S12 with which are associated the following illumination weights: 1, 2, 4, 6, 10, 14, 18, 24, 32, 40, 48 and 56. One effect of the rotating code is to soften the high-weight switchings by reducing the number of weights switched when switching a high weight. To obtain the twelve subscans, a simultaneous scan of two rows is performed for the weights 2, 6, 14 and 24. Such a code furthermore allows a multiple representation of the numbers: 34=32+2=24+10=24+6+4=18+14 +2= . . . etc. This multiple representation of the numbers makes it possible to code the grey levels present on the two rows scanned at the same time in such a way that the weights 2, 6, 14 and 24 are identical. The person skilled in the art may refer to European Patent Application No. 0 874 349 for further details regarding this technique. Although the effect of softening the switching of a high weight is attenuated by the multiple coding, problems of loss of resolution and of contouring still arise when it is not possible to have suitable coding between two simultaneously addressed cells.
In European Patent Application EP-A-0 945 846, it is proposed, among other things, that the use of rotating codes be optimized so as to be able to increase the number of subscans considerably. The technique used consists in decomposing the grey levels GL1 and GL2, for two cells placed on two adjacent rows, into a common value CV and into a specific value SV1 and SV2. Thus, we have:
GL1=SV1+CV,
GL2=SV2+CV,
with, when GL1 less than GL2:
SV1=xcex1*GL1,
SV2=D+xcex1*GL1,
CV=xc2xd(GL1+GL2xe2x88x92SV1xe2x88x92SV2),
and where xcex1 is a coefficient to be defined as a function of the type of code used,
D is equal to the difference GL2xe2x88x92GL1 after rounding, for example rounding to 5.
In most cases, the rounding to 5 of the difference makes it possible to limit the error to plus or minus 1 bit. However, such a system is limited by a deviation in the maximum difference value. It is then necessary to have for example SV1=0, SV2=maximum value, and CV chosen so as to minimize the error. The probability of frequency of occurrence of such a case depends essentially on the choice of the number of subscans specific to each cell and of the duration of illumination of the said subscans.
A second limitation of such a method of coding stems from the dispersion of the various codings for one and the same value. Specifically, the coding variations no longer depend on each cell but on each pair of cells independently of the neighbouring pair. The phenomenon of contouring is strongly attenuated inside the pair of cells but the attenuation of the false contour is less with the neighbouring pairs. The person skilled in the art will note, on reading EP-A-0 945 846, that to minimize this limitation, it is advisable to use a common part which is the largest possible, this having the effect of increasing the probability of error due to the limitation stemming from the deviation in the maximum value.
The invention proposes a novel scanning technique aimed at improving the use of rotating code. The grey level coding method forming the subject of the invention carries out a coding which favours one choice from among two possible codes as a function of the grey levels associated with each cell. The two codes use equivalent criteria so that the disparity between the two codes is minimized. According to the invention, the coding of the highest grey level over the entirety of the subscans has priority. If the coding over the entirety of the subscans is not advisable, the lowest grey level is coded over the subscans common to the two cells of the pair. In both cases, the subscans corresponding to the low illumination weight are favoured. Such a method carries out various codings for one and the same value while retaining great proximity between the various codes.
The subject of the invention is a method of displaying a video image on a plasma display panel for a duration of display, the said panel comprising a plurality of cells arranged in rows and columns, each cell being lit for a duration lying between zero and a maximum display time corresponding to the maximum brightness of a cell for a given brightness setting, the total time of illumination of a cell being divided into several illumination periods corresponding to various subscans among which are distinguished first subscans specific to an addressing of each cell and second subscans common to two cells arranged on neighbouring rows, such that, for a pair of cells sharing the same second subscans, the grey levels GL1 and GL2 of the said cells are decomposed into a common value CV and into a specific value SV1 and SV2 with the aid of the following equation: GL1=CV+SV1 and GL2=CV+SV2, with GL1 greater than or equal to GL2, the said method comprising the following steps:
E1: coding of the highest grey level GL1 over the entirety of the subscans while favouring the subscans whose illumination time is the smallest;
E2: extraction of the specific value VS1 corresponding to the coding of step E1;
E3: coding of the lowest grey level by using the common value CV resulting from step E1 if the specific value SVl extracted in step E2 is greater than the difference GL1xe2x88x92GL2.
In certain cases, the following step is carried out:
E4: coding of the common value CV as being equal to the lowest grey level GL2 if the specific value SV1 extracted in step E2 is less than the difference GL1xe2x88x92GL2, then calculation of a new value SV1=GL1xe2x88x92GL2.
Preferably, if the maximum value encodable with the aid of the first subscans is less than the difference GL1xe2x88x92GL2, then the common value CV is equal to the lowest grey level GL2, and the specific value SV1 is equal to the maximum value encodable on the first subscans.
To control the error due to the use of common subscans, prior to any coding operation, the value 1 may possibly be added to and/or subtracted from one or both grey levels GL1 et GL2 so that the difference GL1xe2x88x92GL2 is a multiple of five.
According to a particular embodiment, the display durations associated with the first subscans correspond to the product of an elementary duration times respectively the factors: 5, 10, 20, 30, 40, 45, and the display durations associated with the second subscans correspond to the product of the elementary duration times respectively the factors: 1, 2, 4, 7, 13, 17, 25, 36.
The invention also relates to a plasma display panel comprising a plurality of cells arranged in rows and columns, each cell being lit for a duration lying between zero and a maximum display time corresponding to the maximum brightness of a cell for a given brightness setting, the total time of illumination of a cell being divided into several illumination periods corresponding to various subscans among which are distinguished first subscans specific to an addressing of each cell and second subscans common to two cells arranged on neighbouring rows, such that, for a pair of cells sharing the same second subscans, the grey levels GL1 and GL2 of the said cells are decomposed into a common value CV and into a specific value SV1 and SV2 with the aid of the following equation: GL1=CV+SV1 and GL2=CV+SV2, with GL1 greater than or equal to GL2, the said panel comprising a grey level encoding device comprising:
a first coding circuit for coding the highest grey level GL1 over the entirety of the subscans while favouring the subscans whose illumination time is the lowest;
a means for extracting a common value CV and a specific value SV1 exiting the first coding circuit;
a selection and calculation circuit for carrying out the coding of the lowest grey level by using the common value CV exiting the first coding circuit if the specific value SV1 extracted from the first coding circuit is greater than the difference GL1xe2x88x92GL2.
The other features of the method are also transposed over to the device.